A spherical detector, such as shown in FIG. 1, generally comprises a first electrode 2 (the cathode) forming a hollow spherical enclosure 6 filled with a gas which is used as an ionisation medium and as an amplification medium. A second electrode 3 (the anode) in the shape of a ball is placed and maintained at the centre of the spherical first electrode 2 by means of a maintaining cane 4.
This type of detector is adapted to detect non-ionising particles such as neutrons, neutrinos, gamma radiation or ionising particles such as electrons, positrons and muons. The gas filling the spherical enclosure is as such chosen so that the particles interact with it in order to create charges that will migrate to the anode and which will be amplified in order to create the signal to be measured.
The first spherical electrode 2 is connected to the mass while the second central electrode 3 is brought to a high positive potential. The detecting is carried out by the ionisation of particles of gas that then produce a positively charged ion and a negatively charged ion. The electrical field applied between the electrodes makes it possible:                to divert the electrons created by the ionisation of the gas to the ball by creating a radial field, and;        to produce an “avalanche” in the vicinity of the ball in order to amplify the signal.        
For example, in prior art, spherical detectors have substantial volumes, of a magnitude of 1 m3. Indeed, in order to increase the detecting space of the particles or radiation, it is necessary to increase the radius of the spherical enclosure 6. This consequently entails substantially increasing the voltage applied to the anode in order to create an electrical field that is strong enough for it to be able to capture the ionised particles far from the centre of the spherical enclosure 6. The increase in the detecting space of the particles also required rigidifying the support of the anode by increasing the diameter of the maintaining cane 4. As it is necessary to have a central anode with a diameter that is greater than that of the maintaining cane 4 (in order to minimise the “shadow” effects of the maintaining cane), this has for incidence an increase in the potential to be applied on the anode.
Moreover, for certain applications, it is interesting to have devices for detecting that are portable, therefore with reduced dimensions. With this type of device of reduced size, it is necessary to apply a lower potential to the anode in such a way as to prevent the phenomena of breakdowns between the two electrodes that are located close to one another. On the other hand, this potential is generally insufficient to create a satisfactory amplification field.